1. Field of the Invention
The present invention relates to a method for the liquid/liquid extraction and recovery of gallium from basic aqueous solutions.
2. Description of the Prior Art
A process for recovering gallium from basic aqueous alkaline solution by means of liquid/liquid extraction is known from the French patent application Nos. 2,277,897, 2,307,047 and 2,307,882. According to this process which is also described and patented in our previously issued U.S. Pat. No. 3,971,843, hereby expressly incorporated by reference and relied upon, gallium present in aqueous alkaline solutions which also contain compounds of aluminum and sodium is recovered by liquid/liquid extraction using substituted hydroxyquinolines. The patented method is especially adapted for the recovery of the gallium present in sodium aluminate lyes originating from the Bayer process, utilizing certain 8-hydroxyquinolines. The gallium recovery method described in the aforementioned patent includes a stage of extraction proper and a stage where the solvent is regenerated and the gallium recovered by means of strong acids. There are several preferred embodiments for carrying out the subject process, depending, on the one hand, on the acid used and its concentration in the stage where the solvent charged with the gallium values is regenerated; on the other hand, the process parameters can be said to depend on the desired purity of the gallium values sought to be recovered.
In the first embodiment, a preferred method of recovering the gallium present in very basic aqueous solutions also containing compounds of aluminum and sodium includes the following stages:
[i] contacting the aqueous solution with a water-insoluble substituted hydroxyquinoline in solution in a water-insoluble organic solvent comprising the halogenated or non-halogenated aliphatic and aromatic hydrocarbons, whereby the gallium and a certain amount of sodium and aluminum values are extracted from the aqueous phase to the organic phase;
[ii] separating the organic phase from the aqueous phase;
[iii] contacting the organic phase with a dilute aqueous solution of an acid to extract the sodium and aluminum from the organic phase, the gallium values remaining in solution in the said organic phase;
[iv] separating the organic phase from the aqueous phase and next contacting the remaining organic phase with a more concentrated aqueous acid solution, to transfer the gallium from the organic phase into the aqueous phase; and thence
[v] separating the gallium values from the aqueous phase.
The acids used are preferably hydrochloric, sulfuric and nitric. The concentration of the dilute aqueous acid solution is preferably from 0.2 M to 0.5 M, and the concentration of the more concentrated aqueous acid solution is preferably greater than 1.6 M. But in those cases where hydrochloric acid is used, the concentration is preferably from 1.3 M to 2.2 M, and more preferably from 1.6 M to 1.8 M.
In a second embodiment, another preferred method of recovering the gallium values comprises the following stages:
[a] contacting the aqueous solution with a water-insoluble substituted hydroxyquinoline in solution in a water-insoluble organic solvent comprising the halogenated or non-halogenated aliphatic and aromatic hydrocarbons, whereby the gallium and a certain amount of sodium and aluminum values are extracted from the aqueous phase to the organic phase;
[b] separating the organic phase from the aqueous phase;
[c] contacting the organic phase with a concentrated aqueous solution of an acid capable of chelating (complexing) the gallium in anionic form, the gallium values remaining in solution in the organic phase while the sodium and aluminum values transfer into the aqueous phase;
[d] separating the organic phase from the aqueous phase and contacting the remaining organic phase with a dilute aqueous acid solution, whereby the gallium values in the organic phase transfer to the aqueous phase; and
[e] separating the gallium values from the aqueous phase.
The acids used are preferably hydrochloric and hydrobromic acid. The concentration of the concentrated acid solution is preferably from 5 M to 8 M, and that of the dilute solution from 1.3 M to 2.2 M.
Further, it too is feasible, if it be acceptable to obtain the gallium values in a less purified state, to conduct the operation in the following seriatim fashion:
[1] contacting the aqueous solution with a water-insoluble substituted hydroxyquinoline in solution in a water-insoluble organic solvent comprising the halogenated or non-halogenated aliphatic and aromatic hydrocarbons, whereby the gallium values and a certain amount of sodium and aluminum values are extracted from the aqueous phase to the organic phase;
[2] separating the organic phase from the aqueous phase;
[3] contacting the organic phase with an aqueous solution of an acid to transfer the gallium, sodium and the aluminum values in the organic phase to the aqueous phase;
[4] separating the aqueous phase from the organic phase; and
[5] recovering the gallium, sodium and aluminum values.
The acid solutions used are preferably either solutions of sulfuric or nitric acids in a concentration of greater than about 1.6 M, or hydrochloric or hydrobromic acids in a concentration from 1.3 M to 2.2 M.
In the gallium extracting stage described in the aforementioned patent, it may be advantageous to add substances with an alcohol function, e.g., heavy alcohols such as N-decanol and isodecanol, and various heavy phenols, as well as various other solvating compounds, e.g., certain phosphoric esters such as tributyl phosphate, in the organic extraction phase, in known manner.
The concentration of substituted hydroxyquinoline in the organic phase is not critical and may vary over wide limits; indeed the same need not be very high, for even a concentration as low as 1% affords good extraction of gallium, due to the far greater affinity of these chelating agents for gallium than for aluminum and sodium; in practice, however, typical concentrations are in the range of from 1% to 50% by volume, with respect to the organic phase, and concentrations of from 6% to 12% have been found to be especially economical.
The topic hydroxyquinolines, moreover, may be employed singly or in any admixture.
Furthermore, although temperature is not a critical parameter for obtaining good results by the method described in our U.S. Pat. No. 3,971,843, it is advantageous for the extraction stage to be carried out at a fairly high temperature, in practice below 100.degree. C., and preferably from 50.degree. to 80.degree. C. It may be added that, in industrial practice, the solutions generally treated are aluminate solutions from the Bayer process, particularly those which are art characterized as "decomposed", which are at a temperature in the vicinity of 50.degree. C., and, although this temperature is less favorable than a higher one, it is nevertheless sufficient to afford satisfactory extraction yields. Moreover, the stage where the solvent is regenerated and the gallium values recovered is carried out in such a way that the organic phase is treated with an acid solution to recover the gallium, at a temperature below that of the extraction stage and preferably near ambient temperature.
The very basic solutions treated by the methods described are notably those in which the OH.sup.- concentration may be as high as 13-14 g ions/l. Thus, the caustic sodium aluminate lyes from the Bayer process, which are preferably treated by this method, generally have a composition corresponding to:
Na.sub.2 O: from 100 to 400 g/l
Al.sub.2 O.sub.3 : from 40 to 150 g/l
while the so-called "decomposed" caustic solutions typically display such compositions as:
Na.sub.2 O: from 150 to 200 g/l
Al.sub.2 O.sub.3 : from 70 to 100 g/l
In industry, installations used for this method are typically assembled as follows. The sodium aluminate solution, with its alumina content reduced following its "decomposition" and the organic phase comprising the selected chelating agent, a solvent and, optionally, the compounds bearing an alcohol function and other solvating compounds are charged into a first counter-current extraction apparatus. A large proportion of the gallium values transfer into the organic phase, the proportion depending upon the respective rates of flow of the two liquids. Aluminum, sodium and certain impurities also transfer into the organic phase. In another extraction apparatus, the resulting organic phase is contacted with a first regenerating solution, comprising a dilute strong acid or a concentrated strong chelating acid; in both instances this leaves virtually only gallium values in the organic phase. The latter organic phase is then treated in a third countercurrent extraction apparatus, whereat it is contacted with a strong acid for the purpose of recovering the gallium, and is then washed with water before the organic phase is recycled into the first countercurrent extraction apparatus. The acid solution, which now includes the gallium values, is then treated to complete its purification; thence the gallium values are extracted therefrom.
Further, a simple commercial installation can be utilized if it be desired to recover less purified gallium values. Such an apparatus may include a first extraction apparatus as described hereinabove, at the outlet of which the organic phase is treated in a countercurrent extraction apparatus where it contacts a solution of a strong acid for the purpose of recovering the gallium, sodium and aluminum.
It had also been found, however, that extended use of alkenyl-8-hydroxyquinoline in a gallium extracting unit by the methods described in the aforementioned patent ultimately resulted in their gradual degradation, thus effecting a reduction in their extracting power.
In our copending application, Ser. No. 824,068, filed Aug. 12, 1977, hereby expressively incorporated by reference and relied upon, there is disclosed a certain group of hydroxyquinolines which were found to be particularly stable, which did not deteriorate over prolonged periods of time, and which retained the noted and notable initial properties of the alkenyl-8-hydroxyquinolines as described in our earlier patent for extracting gallium from sodium aluminate lyes.
The invention disclosed in our 824,068 application reflects an important improvement over the process of our earlier patent for recovering the gallium values contained in very basic aqueous solutions by liquid/liquid extraction, by utilizing an organic phase chiefly consisting of water-insoluble substituted hydroxyquinolines and an organic solvent, characterized in that the substituted hydroxyquinolines are selected from the group consisting of those of the structural formula: ##STR1## wherein each R, which may be the same or different, is an alkyl radical or hydrogen.
In a preferred embodiment of the invention disclosed and claimed in our said 824,068 application, alkyl radicals R has 5 to 20 carbon atoms. Below about 5 carbon atoms, such substituted hydroxyquinolines were generally found to have a solubility which was too great in an acid or basic medium, thus making their use uneconomical due to subsequent losses. Above about 20 carbon atoms, the substituted hydroxyquinolines of the 824,068 application have a viscosity which makes them quite difficult to use.
In another preferred embodiment of the invention of the 824,068 application, the radical R is located in the 7-position on the 8-hydroxyquinoline ring, and those particular preferred compounds have the structural formula: ##STR2## wherein n is preferably between 5 and 20.
The hydroxyquinolines of our copending application enable the gallium values present in very basic aqueous solutions, particularly in sodium aluminate lyes emanating from the Bayer process, to be almost totally extracted without any loss over prolonged periods of time.
It has been further found that in the case of highly basic solution even though the essential cause of the degradation of the 8-hydroxyquinolines is the highly basic nature of the medium to be extracted, (1) the ambient air (oxygen) oxidation thereof also contributes significantly to the degradation phenomenon over prolonged periods of use, and (2) such oxidative degradation due to the oxygen in the ambient air affects the different 8-hydroxyquinolines to varying or different degrees.
In our copending application Ser. No. 923,644, filed July 11, 1978 now U.S. Pat. No. 4,169,130, hereby expressively incorporated by reference and relied upon, there is disclosed a further improvement of the process described in our U.S. Pat. No. 3,971,843, by means of which not only do the gallium values continue to be essentially wholly recovered from highly basic aqueous solutions thereof via our patented method and the method of our 824,068 copending application, but also the loss of hydroxyquinoline through oxidative degradation is markedly diminished. This improvement comprises conducting at least the gallium extraction step of the topic liquid/liquid extraction under an inert, gaseous atmosphere. Also according to an embodiment of the improvement the entire liquid/liquid extraction is advantageously carried out under an inert atmosphere, and, as representative examples of suitable inert gases defining the inert atmosphere, there are mentioned argon, nitrogen, and the like.